A voltage controlled amplifier amplifies an input signal according to the level of a control signal. The VCA is often used in audio consoles in order to control the gain of an input audio signal. By having the audio signals pass through separate VCAs, the levels of the various audio signals, and therefore the mix of the signals, may be selectively controlled by varying the control voltages applied to the respective VCAs. Also, the control voltage may be varied in order to fade the audio signal either in or out. For example, to perform a fade operation, an input audio signal may be fully attenuated with the VCA (zero gain) or amplified to a desired level. In many operations, it is desirable to adjust the gain from full attenuation to unity gain.
Typically, the control signal supplied to the VCA is output from a "VCA-taper" potentiometer (a "fader"). The fader may be manually adjusted to permit direct control of the VCA's processing of the input audio signal. Alternatively, the fader signal may be supplied automatically by other switching devices. A control circuit in which two manually adjusted faders are used to control a voltage control signal input to a VCA is described in commonly-owned U.S. Pat. No. 5,317,641 entitled "Fader Depth Control Apparatus" (Attorney Docket SOA-010). The control circuit of this application has been applied so as to supply a 0 to 5 VDC control level to a DBX 2155 VCA.
A newer and particularly useful VCA device for amplifying an input audio signal under control of a fader voltage is available from Analog Devices as model number SSM-2018. According to the manufacturer's specifications, this component has a characteristic attenuation law of -28 mV/dB. Thus, this relationship determines the particular control voltage required to be input in order to achieve the desired level of attenuation of an input signal.
A recommended application circuit 1 for the SSM-2018 is shown in FIG. 1. Among the other components shown, this circuit includes a time constant comprised of resistors 2 and 2A and a capacitor 4. Together these elements provide a time constant of approximately 5 ms in the control signal path which leads to the control voltage pin 11. This time constant is provided so as to suppress audible noise and distortion. The manufacturer further recommends the use of a larger capacitor (of 10 .mu.F to 20 .mu.F) to improve low frequency distortion. Such capacitors increase the time constraint to between approximately 50 ms to 100 ms.
When such a simple time constant is coupled to the control signal path as shown, it limits the bandwidth of the control signal. This is undesirable in many applications where a higher-frequency control signal is used. For example, the recommended time circuit may limit operation where the control signal is varied automatically at relatively high speeds. While the time constant may be changed to improve speed, this may result in increased levels of distortion and noise being introduced into the VCA circuitry thereby limiting performance.
Accordingly, there is a need for an improved circuit for providing a control signal to a VCA, such as the SSM-2018, to thereby adjust an input audio signal between full attenuation and unity gain.
There is an additional need to ensure that such an improved control circuit may be manufactured economically. Specifically, there is a need that relatively few components be used, and that such components be inexpensive and widely available.